A barrel-related interneuron in layer 4 of rat somatosensory cortex with a high intrabarrel connectivity.

Koelbl C, Helmstaedter M, Lübke J, Feldmeyer D - Cereb. Cortex (2013)

Bottom Line:
Three distinct clusters of FS L4 interneurons were identified based on their axonal morphology relative to the barrel column suggesting that these neurons do not constitute a homogeneous interneuron population.We found on average 3.7 ± 1.3 putative inhibitory synaptic contacts that were not restricted to perisomatic areas.In conclusion, we characterized a novel type of barrel cortex interneuron in the major thalamo-recipient layer 4 forming dense synaptic networks with L4 spiny neurons.

BHT263F2: Quantitative identification of different FS L4 interneuron types. (A) Reconstruction of a biocytin-labeled L4 interneuron located in the center of a cortical barrel. Dendrites, red; axons, light blue. The home barrel of this interneuron is outlined in yellow, the home “barrel column” in light yellow. The 2 neighboring barrels are shown in light gray. (B) To classify L4 interneurons, the fractions of the axon within a barrel and within the home column were used. Cluster 1 interneurons are in green, cluster 2 interneurons in red and those of cluster 3 in blue (see panel C). (C) Based on these 2 parameters (axon–barrel ratio and axon–column ratio), a cluster analysis was performed that resulted in 3 distinct interneuron types: Cluster 1 interneurons had a relatively low axon–barrel ratio but generally high axon–column ratios, Cluster 3 interneurons had highly local axons that were confined to their home barrel and thus had both high axon-barrel and axon-barrel ratios. Cluster 2 L4 interneurons were intermediate.

Mentions:
Whole-cell patch-clamp recordings and simultaneous biocytin labeling was performed for a total of 45 interneurons in layer 4 of the barrel cortex of 18- to 22-days-old rats (Fig. 1). The dendritic and axonal arborizations of these neurons were completely reconstructed for a subsequent quantitative morphological analysis. The total axonal length was measured in 3D and the fractions of axonal length in different functional compartments (home barrel, home barrel column, adjacent column, e.g., see Fig. 2A) were determined. Data were derived from Neurolucida reconstructions of biocytin-filled neurons, which were aligned to the center of the home barrel. We took bright-field photographs of the slice after recording, which showed visible barrel fields as well as somata positions indicated by the tips of patch pipettes (Fig. 1A). The alignment was done by matching soma positions and pia mater of the reconstructions with the corresponding bright-field photomicrographs (see Materials and Methods section for further details).Figure 2.

BHT263F2: Quantitative identification of different FS L4 interneuron types. (A) Reconstruction of a biocytin-labeled L4 interneuron located in the center of a cortical barrel. Dendrites, red; axons, light blue. The home barrel of this interneuron is outlined in yellow, the home “barrel column” in light yellow. The 2 neighboring barrels are shown in light gray. (B) To classify L4 interneurons, the fractions of the axon within a barrel and within the home column were used. Cluster 1 interneurons are in green, cluster 2 interneurons in red and those of cluster 3 in blue (see panel C). (C) Based on these 2 parameters (axon–barrel ratio and axon–column ratio), a cluster analysis was performed that resulted in 3 distinct interneuron types: Cluster 1 interneurons had a relatively low axon–barrel ratio but generally high axon–column ratios, Cluster 3 interneurons had highly local axons that were confined to their home barrel and thus had both high axon-barrel and axon-barrel ratios. Cluster 2 L4 interneurons were intermediate.

Mentions:
Whole-cell patch-clamp recordings and simultaneous biocytin labeling was performed for a total of 45 interneurons in layer 4 of the barrel cortex of 18- to 22-days-old rats (Fig. 1). The dendritic and axonal arborizations of these neurons were completely reconstructed for a subsequent quantitative morphological analysis. The total axonal length was measured in 3D and the fractions of axonal length in different functional compartments (home barrel, home barrel column, adjacent column, e.g., see Fig. 2A) were determined. Data were derived from Neurolucida reconstructions of biocytin-filled neurons, which were aligned to the center of the home barrel. We took bright-field photographs of the slice after recording, which showed visible barrel fields as well as somata positions indicated by the tips of patch pipettes (Fig. 1A). The alignment was done by matching soma positions and pia mater of the reconstructions with the corresponding bright-field photomicrographs (see Materials and Methods section for further details).Figure 2.

Bottom Line:
Three distinct clusters of FS L4 interneurons were identified based on their axonal morphology relative to the barrel column suggesting that these neurons do not constitute a homogeneous interneuron population.We found on average 3.7 ± 1.3 putative inhibitory synaptic contacts that were not restricted to perisomatic areas.In conclusion, we characterized a novel type of barrel cortex interneuron in the major thalamo-recipient layer 4 forming dense synaptic networks with L4 spiny neurons.